Mixin inheritance order problem with BaseModel

[ocktowen]

I have been doing some experimentation with this package and have found some bugs here and in the base xsdata one. I have created issues for each of them and a repository with instructions and examples.

Trying to define an extension of type class to create a mixin doesn't work as expected because if the class inherits BaseModel directly there is no way to put the mixin at the beginning of the base list.

Thank you for the work on this.